diesel filter 306 estate

Which variety of diesel is it? It may make a difference. The 106 filter (on a 1.5 diesel) screws into the thermostat housing on the passenger side of the engine. On a 1.9 Xantia/BX (i.e. a TUD engine I believe it's called) it's also in or on the stat housing but more accessible being on the front of the engine in a sort of black pot with 4 allen type screws holding the lid on.

Assuming it's the 1.9 D then it is indeed on top of the thermostat housing, RHS of the block as you look at is from the front. If it happened to be an early HDI then it is centre front of the block under the engine top cover. In both cases you can see the fuel pipes connected to it, and it is a black plastic cylender about 100mm dia.